1. Field of the Invention
This invention relates to a method for controlling fungi, insects, rodents, nematodes and weeds, and for inhibiting nitrification in a soil environment.
2. Description of the Art
Among the more economically serious plant parasites are nematodes, roundworms comprising as many as 10,000 species, of which at least 150 are known to adversely affect plant life. Plant parasitic nematodes have been known since about 1750. Most of the nematodes which cause crop damage do so by feeding on plant roots and, therefore, are found primarily in the upper few inches of soil in the roots or in close proximity to the roots. Nematode feeding causes hypertrophy or gall formation, and the evidence of heavy infestation is plant stunting, pale foliage, wilting, and even plant death in extreme cases.
Virtually all of the world's crops and ornamental plants can be attacked by parasitic nematodes. Important destructive nematode species include the root knot nematodes, which are hosted by tomatoes, alfalfa, cotton, corn, potatoes, citrus and many other crops, the golden nematode of potatoes, the sugar beet cyst nematode and the citrus nematode. These, and a few other species, are described in "The Soil Pest Complex", Agricultural and Food Chemistry, Vol. 3 pages 202205 (1955). Also described therein is a further complication resulting from nematode infestation, namely, a lowered resistance to the effects of plant attack by bacteria and pathogenic soil fungi.
Except for small volumes of soil which can be sterilized, it has not been found possible to eliminate nematodes. Parasite populations can, however, be kept at levels which economically permit agricultural operations by soil fumigation, crop rotation using nonhosting plant varieties, and (to a much lesser extent) the development of plants which are resistant to infestation. In many instances, control of nematodes is achieved only by combinations of these techniques, and most control programs have proven quite costly.
The process of soil fumigation requires the movement of gaseous chemicals through the soil which is treated, and the readily apparent necessity for a sufficient concentration of gas at a given temperature and pressure condition to be lethal to the pest which would be controlled. Volatility of the chemical agent is critical to successful fumigation, since a very volatile substance will disperse too readily and not develop an effective concentration except for locations very close to the point of introduction to the soil. Substances having a very low volatility are also undesirable, since they will not disperse in the soil, and will be effective only at locations near the point of introduction.
Carbon disulfide is the first reported soil fumigant, used in Europe during the 1870's to control the sugar beet nematode This agent is commercially impractical, however, since very large quantities must be applied (due to the high volatility) and the material is quite flammable, reportedly being ignited even by static electricity resulting from pouring the material out of drums. In addition, carbon disulfide possesses a very objectional odor, and its vapors are toxic to humans. When sold for fumigant use, the carbon disulfide is normally mixed with an inert fire retarding compound, such as carbon tetrachloride, and occasionally also with another fumigant. Typically, these compositions do not contain over about 20 percent by weight of carbon disulfide.
In addition to soil uses, carbon disulfide has been proven effective in the fumigation of commodities, as an insecticide, as a rodenticide, and for controlling certain weeds. W.T. Thomson, in Agricultural Chemicals Book III Miscellaneous Chemicals, 1976-77 Revision (Thomson Publications, P.O. Box 7964, Fresno, Calif.), describes application methods to grain entering storage and to stored grains, and the chamber fumigation of commodities, at pages 1112. Examples are also given of methods for controlling apple tree borers and soil living rodents. Carbon disulfide is approved by the U.S. Environmental Protection Agency as an insecticide, when used as a fumigant after harvest for barley, corn, oats, popcorn, rice, rye, sorghum (milo), and wheat. Carbon disulfide is also useful as a nematocide for other preplanted (fallow) soil. The amount of carbon disulfide, recommended for this use, would be phytotoxic if applied to growing plants.
Numerous compositions possessing nematocidal properties have been developed, including active ingredients such as the polyamines of U.S. Pat. No. 2,979,434 to Santmyer, the heterocyclic compounds of U.S. Pat. No. 2,086,907 to Hessel, and various halogenated compounds. Among the useful halogen-containing nematocides are 1,2-dibromoethane, methyl bromide, 3-bromopropyne, 1,2-dichloropropane, ethylene dichloride and others, all of which are quite phytotoxic, therefore restricting their utility to mostly preplanting treatments.
One compound which enjoyed considerable commercial success is 1,2-dibromo-3-chloropropane DBCP), which can be used to control nematodes in soils with growing perennial plants. However, use of this material has been limited due to a finding of undesirable reproductive system effects in workers exposed to the chemical, and the possibility that the compound is a carcinogen. The unavailability of DBCP has been a serious setback to growers of perennial crops, such as grapes, stone fruits and nuts, since these crops experience more severe cumulative nematode population increases, and most replacement soil fumigants are phytotoxic. U.S. Pat. Nos. concerned with the use of DBCP as a soil fumigant include 2,937,936 to Schmidt and 3,049,472 to Swezey.
A further class of materials which have been utilized to control nematodes is the thiocarbonates. U.S. Pat. No. 2,676,129 to Pashour describes the preparation of lower aliphatic disubstituted trithiocarbonates having the structure as in (1): ##STR1## wherein R.sub.1 and R.sub.2 are alkyl radicals having from three to nine carbon atoms. The compounds were dissolved in acetone and added to nematode-infested soils, resulting in control of the nematodes.
Other compounds have been reported by Seifter in U.S. Pat. Nos. 2,836,532 and 2,836,533, the former relating to the use of sodium and potassium trithiocarbonate, and the latter pertaining to alkali metal and ammonium salts of tetrathioperoxycarbonic acid. Both are described as effective in nematode control.
These references state that "not all carbon disulfide derivatives are effective nematode toxicants." Furthermore U.S. Pat. No. 2,836,532 points out that sodium trithiocarbonate is unexpectedly superior to potassium trithiocarbonate as a nematocide.
Another serious problem in agriculture is that of low nitrogen use-efficiency, since crops have been found to recover only 30 to 70 percent of the total amount of expensive fertilizer nitrogen which is applied to the soil. Most of the lost nitrogen is due to nitrite and nitrate ions, which are exceptionally mobile in a soil environment, and therefore are readily lost by surface runoff and also by leaching from the plant root zone into deeper soil. Other losses of these ions are due to denitrification, which is reduction to elemental nitrogen or gaseous nitrogen oxides under conditions of limited aeration. In addition to the direct economic losses, these nitrogen forms constitute environmental pollutants when runoff enters surface and ground water systems.
Although some nitrogen is applied to soil in the form of nitrate (e.g., ammonium nitrate-containing fertilizers), most nitrogen fertilization is with ammonia, ammonium compounds other than nitrate and urea materials. Ammonium nitrogen is fairly tightly bound by various physical and chemical processes in a soil environment and, therefore, is much less subject to losses. Unfortunately, the bound ammonium nitrogen is also less available to plants.
The process of nitrification results in conversion of ammonium ions into nitrate ions. Microbial species known as nitrosomonas oxidize ammonium to nitrate; nitrobacter species oxidize nitrite to nitrate. This more mobile ion is easily taken up by plant roots and is also readily assimilated by plants. In this regard, the nitrification process is desirable, but control of the rate at which conversion occurs has not been easily obtained. Inhibition of the nitrification would tend to make the applied nitrogen available to plants over a longer period of time, resulting in an increased plant uptake efficiency.
Various compositions have been offered as inhibitors of nitrification, including expensive organic materials such as 2-chloro-6-(trichloromethyl)-pyridine, 2-amino-4-chloro-6-methyl-pyrimidine, sulfathiazole, alkanolysulfathiazoles, and others. A paper by J. M. Bremner and L. G. Bundy in Soil Biology and Biochemistry, Vol. 6, pages 161-165 (1974) describes the efficacy of various volatile organic sulfur compounds, including methyl mercaptan, dimethyl sulfide, dimethyl disulfide, carbon disulfide, and hydrogen sulfide. Carbon disulfide in very small amounts is described as having "a remarkable inhibitory effect on nitrification of ammonium in soils incubated in closed systems". Carbon disulfide was tested in the field by J. Ashworth et al., Chemistry and Industry, Sep. 6, 1975, pages 749-750, and found to be effective as a nitrification inhibitor. Hawkins, in U.S. Pat. No. 4,078,912, describes the use of sodium, potassium and ammonium trithiocarbonates, and of xanthates, either alone or in fertilizer mixtures, to inhibit nitrification; the mode of operation is attributed to a release of carbon disulfide by the compounds.
One additional potential problem, which could be presented to the agricultural industry in the very near future, is the loss of the widely used, effective fumigant, 1,2-dibromoethane, due to environmental concerns. This agent is approved for use on the same crops as is carbon disulfide, and is additionally used extensively in chambers for fumigating fruits and vegetables to control various insects.
The chemistry of thiocarbonic acids and salts has been studied in some detail, as indicated in the papers by O'Donoghue and Kahan, Journal of the Chemical Society, Vol. 89 (II), pages 1812-1818 (1906); Yeoman, Journal of the Chemical Society, Vol. 119, pages 38-54 (1921); and Mills and Robinson, Journal of the Chemical Society, Vol. 1928. (II), pages 2326-2332 (1928). According to O'Donoghue and Kahan, derivatives of thiocarbonic acid were prepared by Berzelius, who reacted aqueous solutions of hydrosulfides with carbon disulfide, the reactions occurring as in (2): EQU 2 KHS+CS.sub.2 K.sub.2 CS.sub.3 +H.sub.2 S (2)
giving unstable solutions which yielded unstable crystalline salts.
Other thiocarbonates were prepared and further characterized by O'Donoghue and Kahan. Their paper, at page 1818, reports the formation of ammonium thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic acid, prepared by dropping a solution of `calcium thiocarbonate` into concentrated hydrochloric acid. The `calcium thiocarbonate` obtained by the authors is reported to be a double salt, including the calcium cation in combination with both hydroxide and trithiocarbonate anions.
The noted paper by Yeoman reports the further study of thiocarbonates (called trithiocarbonates therein) and also reports the preparation and properties of perthiocarbonates (or tetrathiocarbonates), derivatives of tetrathiocarbonic acid, H.sub.2 CS.sub.4. Yeoman prepared various thiocarbonate salts including alkali metal and alkaline earth metal tri- and tetrathiocarbonates, as well as the ammonium salts. It is noted by Yeoman that the ammonium thiocarbonates are very unstable, which would argue against their use as fumigants.
Considerable explanation is provided concerning the stability of the other thiocarbonates, with the alkali metal thiocarbonates exemplified by sodium and potassium trithiocarbonates and perthiocarbonates. Sodium trithiocarbonate solutions in water are said to remain stable only if oxygen and carbon dioxide are "rigidly excluded"; the presence of oxygen causes decomposition to form carbon disulfide and thiosulfates, while carbon dioxide decomposes the solution to give a carbonate and carbon disulfide. Similarly, solutions of sodium perthiocarbonate are reported to be stable for a considerable time in the absence of oxygen, the presence of air causing decomposition into thiosulfate and carbon disulfide, while carbon dioxide decomposes the compound to form a carbonate, elemental sulfur, carbon disulfide, and hydrogen sulfide. The potassium thiocarbonates behave similarly, according to Yeoman.
Yeoman also attempted to prepare and characterize for stability thiocarbonate salts of four of the alkaline earth metals. Yeoman was unable to prepare a "pure" calcium tri- or tetrathiocarbonate, but observed that the double salt of calcium trithiocarbonate that he prepared was more stable (probably because it was less hygroscopic) than the sodium or potassium thiocarbonates. The Barium tetrathiocarbonate could not be isolated (although Yeoman felt that it existed in solution). Barium trithiocarbonate was found to be stable although it was alleged to behave like sodium trithiocarbonate when dissolved in water. The preparation of aqueous solutions of the tri- and tetrathiocarbonate of magnesium and strontium was alleged (the magnesium thiocarbonates were not characterized), however the stability of any of the magnesium or strontium salts or solutions was not determined.
In view of the above, it is clear that the chemical behavior of the alkaline earth metal thiocarbonate salts is unpredictable. Moreover, it is clear that there is no method taught in the art for preparing either the trithio- or tetra thio-salt of calcium.
A need exists for a fluid which can release carbon disulfide for fumigation and nitrification inhibiting purposes, but which can be stored and handled safely and without significant loss of effectiveness during a reasonable commercial storage and delivery cycle.
It is therefore an object of the present invention to provide a stabilized liquid composition which can be caused to release fumigants, including carbon disulfide.
It is a further object to provide a stabilized composition which is miscible with water to form a fumigant and nitrification inhibitor which can be applied to soils by means of fluid handling equipment or introduced into irrigation water.
Other objects and advantages of the instant invention will be apparent from a careful reading of the specification below.